(1) Field of the Invention
This invention relates to a pneumatic tire, and more particularly to an improvement in a rubber reinforcing layer as an outermost bead rubber suitable for use in a pneumatic tubeless radial tire for heavy vehicles.
(2) Description of the Prior Art
In a pneumatic tubeless radial tire for heavy vehicles, a carcass composed of at least one ply containing mainly metal cords arranged in the radial direction of the tire is turned around each bead core from the inside to the outside to form a turnup portion. A stiffener having a substantially triangle shape in section is disposed between the carcass and its turnup portion. A cord-reinforcing layer composed of metal cords and/or organic fiber cords is disposed axially outside the turnup portion. Further a rubber reinforcing layer for improving the resistance to rim-chafing is disposed as a bead rubber at the outermost side of the cord reinforcing layer. The rubber reinforcing layer extends from the axially outside of the tire to the axially inside thereof through a bead heel portion, and is connected to a lower area of an inner liner at a position radially outside an edge portion of a bead toe to form a bead portion.
The structure of the rubber reinforcing layer, is generally one of two types, one of which being the case where the rubber reinforcing layer contacts with the carcass turnup portion and is covered with the inner liner, and the other being the case where the rubber reinforcing layer covers the lower portion of the inner liner, which mainly result from the difference between the manufacturing systems.
In any case, however, the rubber reinforcing layer is composed of a single rubber composition having rubber characteristics which are selected mainly for the purpose of improving the resistance to rim-chafing against a rim flange to be located axially outside this layer.
However, such an arrangement of the rubber reinforcing layer is apt to cause the breaking of the bead toe portion, which particularly results from the heat deterioration after use and is frequently produced when the tire is disassembled from and reassembled onto the rim.
With respect to the above latter arrangement in which the rubber reinforcing layer covers the lower portion of the inner liner, in addition to the breaking of the bead toe portion as mentioned above, a large number of cracks are produced at the axially inner portion of the rubber reinforcing layer by combining the feature that the heat deterioration takes place over a wide range particularly at the axially inner portion with the repeated stress concentration during the running of the tire under a load. Further, the adhesive force between the lower portion of the inner liner and the rubber reinforcing layer is lowered by the combination of the heat deterioration and the repeated stress concentration as described above, resulting in the occurrence of the peeling phenomenon therebetween.
When causing bead toe breaking, crack formation and/or peeling phenomenon, the air-tightness of the tire internal chamber filled with air is disadvantageously damaged, or if water remains in the tire internal chamber during the rim-assembling, water penetrates into the inside of the tire to produce the rusting of the metal cord, resulting in the cord breaking-up.